In a sputter deposition process, a substrate is placed in a vacuum chamber filled with a gas such as argon, termed “sputtering gas”, at a low pressure. The material to be sputtered, termed “target”, is disposed near the substrate and is electrically connected to a negative electrode, or cathode. A positive electrode, or anode, is disposed nearby within the vacuum chamber. A high negative voltage, between −100 and −1000 Volts, is applied to the cathode, causing ionization of the sputtering gas and a plasma discharge formation above the cathode target. Positively charged sputtering gas ions bombard the negatively charged cathode target, causing atoms of the target to be thrown (sputtered) in space, fly towards the substrate, and adhere to it. In a variety of sputter deposition called reactive sputter deposition, a reactive gas, such as oxygen, is further provided near the substrate surface to immediately enter into a chemical reaction with the freshly adhered atoms, forming a chemical compound film, such as an oxide film. A metal target can be used to sputter metal atoms. When oxygen oxidizes atoms adhered to the substrate, a metal oxide film is formed.
In an endeavor to attain increased deposition rates and lower operating pressures, magnetically enhanced targets have been used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field causes the electrons to travel in a closed loop, commonly referred to as a “race track”, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability thereby leading to a much higher sputtering rate than that obtained without the use of magnetic confinement. Furthermore, the sputtering process can be accomplished at a much lower gas pressure.
The introduction of reactive gas into a sputter deposition chamber is known to cause problems. The reactive gas reacts not only with the deposited film but also with the exposed metal of the target, oxidizing the target metal and reducing the sputtering efficiency. Because of this, the reactive gas concentration has an upper practical limit. The reactive gas concentration limit, in its turn, imposes an upper limit on the sputtering rate, because when the sputtering rate is too high, the reactive gas does not react with all sputtered atoms, causing a degradation of the deposited film. As a result, reactive sputter deposition is almost always a much slower process than a corresponding non-reactive sputter deposition.
Reducing reactive gas flow, increasing sputtering gas flow, or increasing cathode power increases the deposition rate in a reactive sputtering process, but results in a higher optical absorption in the deposited film, due to the presence of partially unoxidized metal in the film. The lack of oxidation can be only partially compensated with an higher oxygen flow, because higher oxygen flow rates cause a deposition rate decrease due to reduced sputtering efficiency, as explained above.
Various solutions to the slow deposition rate of reactive metal compounds have been proposed. For example, Scobey et al. in U.S. Pat. No. 4,851,095 disclose a “partial pressure” technique to decouple the deposition and oxidation processes, i.e. to prevent the oxidation gas from entering the deposition zone by creating a dynamic pressure differential of the reactive gas between the deposition and oxidation zones. To achieve this goal, both deposition and reaction zones are made long and narrow, and are disposed adjacent the periphery of a moving substrate carrier in the form of a vertical drum.
Chaplin et al. in European Patent Application EP 0970261 A1 disclose a method and apparatus for sputter deposition of metal oxides or other compounds at enhanced rates, in which the sputter deposition target is placed at a greater than usual distance from the substrate, while the target metallic erosion track is confined to a narrower width than is typical in prior art systems. By reducing the width of the erosion path of the target, the ratio of reacted metal to unreacted metal in the erosion path can be reduced. Moving the target significantly further away than usual from the substrate reduces the deposition rate per unit area on the substrate. This decrease in deposition rate provides additional time for the reaction to occur on the substrate where the reaction is desirable. At the same time, the total area over which deposition is occurring is increased. By increasing the distance between the target and the substrate and reducing the deposition rate per unit area, but increasing the area where deposition occurs, the film thickness on a substrate moving adjacent to the target remains substantially the same, but more reaction occurs at the film.
Detrimentally, the approaches of Scobey and Chaplin require very large vacuum chambers. Furthermore, the reduced width of the erosion target in Chaplin deposition system can potentially reduce coating uniformity, depending on the geometry used. Narrower erosion tracks reduce target utilization; also, narrow erosion tracks can increase power density at the target, which could lead to cracking of the target or even de-bonding due to larger temperature gradients in the target.